An electroluminescent device (hereinafter may be referred to as an organic EL device) is a surface light emitting device that has a laminated structure, in which an organic luminescent layer is provided between an anode and a cathode, and is a self- luminescent device that uses the luminescence when holes injected from the anode and electrons injected from the cathode are recombined inside the luminescent layer.
Researches and developments for such organic EL devices have been actively conducted for solving various technical problems. One of them is a research and development to increase the efficiency of recombination of the electrons and holes, that is, to increase the current efficiency for luminescence by optimizing the balance (carrier balance) of the number of electrons and the number of holes that are recombined in the organic luminescent layer. As a conventional technique for optimizing the carrier balance of the number of electrons and the number of holes, insertion of a charge injection layer, a charger transport layer, and a charger inhibition layer (also known as a charger block layer) is known.
A method for inserting the charge injection layer inserts, as a hole injection layer or an electron injection layer, a material layer having appropriate HOMO (Highest Occupied Molecular Orbital) or LUMO (Lowest Unoccupied Molecular Orbital), such that a charge of electrons or holes, whichever less, is injected smoothly from an electrode. The method increases efficiency of recombination of the electrons and holes by optimizing the balance of the number of carriers injected in the organic luminescent layer. Moreover, a method for inserting the charge transport layer inserts a layer of a material having hole transportability or electron transportability as the hole transport layer or the electron transport layer, respectively, between the above-discussed charge injection layer and the organic luminescent layer. The method compensates the difference between the hole mobility and the electron mobility of the organic luminescent layer by adjusting the thickness of the material layer and controlling the time for the carrier to reach the organic luminescent layer, and increases the efficiency of recombination of the charges in the organic luminescent layer. Furthermore, to increase the efficiency of recombination, the method for inserting the charge block layer inserts, as a hole block layer (hole blocking layer) or an electron block layer (electron inhibition layer), a material layer having appropriate HOMO or LUMO for suppressing a charge of the electrons or the holes having a higher number than the other, or a charge of the electrons or the holes having a higher mobility than the other, from passing though the organic luminescent layer.
In addition, for the organic luminescent layer structured by host materials and a dopant, an organic luminescent layer, in which the dopant is structured by a luminescent compound and a non-luminescent compound, and in which a band gap for the non-luminescent compound is larger than a band gap of the luminescent compound so as to increase the efficiency of recombination, has been proposed (e.g., see Patent Document 1). Moreover, similarly, an organic EL device that is structured such that, of a first dopant that has a shorter wavelength than a fluorescence peak wavelength of the host material and a second dopant that has a longer wavelength than the fluorescence peak wavelength of the host material, the second dopant emits the light, has been proposed (e.g., see Patent Document 2). Further, as a method to control the mobility of the carrier, addition of a current acceleration material in order to lower the voltage has been proposed (e.g., see Patent Document 3).
As discussed above; a few methods to increase the efficiency of recombination have been proposed. However, when adapting the organic EL device to a full-color display consisting of R (red), G (green) and B (blue), it is generally necessary to insert a hole blocking layer, a charge transport layer and the like, as well as to change an device structure of the material structuring the hole blocking layer, the charge transport layer and the like, such as a type of the materials or the optimal film thickness, with respect to each of colors for RGB.
As one of techniques to provide colors to the organic EL device, there is a method to color the organic luminescent layer with, for example, R (red), G (green) and B (blue). This method forms a luminescent layer by coloring luminescent materials with respective colors for emission in such colors, in order to achieve the luminescence with the respective luminescent colors. Therefore, it is possible to color the luminescent material highly precisely with R (red), G (green) and B (blue) without a color filter or CCM (color changing material) as a required structure the organic luminescent layer. Accordingly, it is a technique for coloring the organic EL device, which allows the organic EL device to be thinner, reduce costs, and to achieve excellent light usage efficiency. A vapor deposition method and a coating method are widely used as a method for forming the luminescent layer by the coloring method. As the vapor deposition method, there is a coloring vapor deposition method using a high definition mask. As the coating method, there are an inkjet method, a gravure method, a spray method, a photolithographic method and the like.
By the vapor deposition method, although a luminescent layer that has excellent luminescent driving life and luminescence efficiency, can be formed because lamination and high purification of the materials structuring the organic EL device can be relatively easy, there is a problem that this formation method is highly costly. The reason is that an expensive vacuum vapor deposition device that has an alignment mechanism to vapor-deposit the luminescent material of the respective color at predetermined regions, a highly precise and expensive mask for vapor-depositing the luminescent materials of the respective color at predetermined regions, are needed and the like. These cause an increase in equipment investment and a decrease in yielding. On the other hand, for the formation of the luminescent layer by the coating method, the equipment investment is relatively inexpensive compared with the vacuum vapor deposition device used in the vapor deposition method, and the area of a panel can be increased. Therefore, as a production method, the coating method is advantageous over the vapor deposition method.    [Patent Document 1] Japanese Laid-Open Patent Application No. 2003-68466    [Patent Document 2] Japanese Laid-Open Patent Application No. H9-134789    [Patent Document 3] Japanese Laid-Open Patent Application No. 2003-68461